This application is a 371 of PCT/SE00/01703 Sep. 7, 2000.
This invention relates to a lubricant for metallurgical powder compositions, as well as a metal-powder composition containing the lubricant. The invention further concerns a method for making sintered products by using the lubricant, as well as the use of the lubricant in a metal-powder composition in warm compaction.
In industry, the use of metal products manufactured by compacting and sintering metal-powder compositions is becoming increasingly widespread. A number of different products of varying shapes and thickness are being produced, and the quality requirements placed on these products are continuously raised. Thus, it is of paramount importance that the manufactured metal products have high and consistent density.
In metal compaction, different standard temperature ranges are used. Thus, cold pressing is predominantly used for compacting metal powder (the powder has room temperature). Use is also made of hot isostatic pressing (HIP) and warm pressing (compaction at temperatures between those used in cold pressing and HIP). Both cold pressing and warm pressing require the use of a lubricant.
Compaction at temperatures above room temperature has evident advantages, yielding a product of higher density and higher strength than compaction performed at lower pressures.
Most of the lubricants used in cold compaction cannot be used in high-temperature compaction, since they seem to be effective within a limited temperature range only. An ineffective lubricant considerably increases the wear of the compacting tool.
How much the tool is worn is influenced by various factors, such as the hardness of the material of the tool, the pressure applied, and the friction between the compact and the wall of the tool when the compact is ejected. This last factor is strongly linked to the lubricant used.
The ejection force is the force required for ejecting the compact from the tool. Since a high ejection force not only increases the wear of the compacting tool but also may damage the compact, this force should preferably be reduced.
However, the use of a lubricant may create problems in compaction, and it is therefore important that the lubricant is well suited to the type of compaction carried out.
In order to perform satisfactorily, the lubricant should, when subjected to pressure and shear forces, deform and flow between the particles of the powder composition. Some of the lubricant will be forced into the gap between the compact and the tool, thereby lubricating the walls of the compacting tool. By such lubrication of the walls of the compacting tool, the ejection force is reduced and thus, the wear of the die.
Another reason why the lubricant has to deform and exhibit flow when subject to pressure and shear forces during the compaction is that it would otherwise result in pores upon sintering of the compact. It is well known that large lubricant particles will result in large pores, which will have adverse effects on the dynamic strength properties of the product.
U.S. Pat. No. 5,154,881 (Rutz) discloses a method for making sintered products on the basis of a metal-powder composition containing an amide lubricant. Apart from the lubricant, which consists of the reaction product of a monocarboxylic acid, a dicarboxylic acid and a diamine, the composition contains iron-based powder. The amide lubricant thus consists of an amide product mixture chiefly made up of diamides, monoamides, bisamides and polyamides (cf column 4, lines 55-56). Especially preferred as a lubricant is ADVAWAX(copyright) 450 or PROMOLD(copyright) 450, which is an ethylenebisstearamide product and which is also the only lubricant tested. When industrially used this lubricant requires an accurate temperature control in order to perform satisfactorily. If the powder temperature exceeds the temperature set for the warm compaction, usually about 130xc2x0 C. production problems may occur.
Furthermore, U.S. Pat. No. 4,955,798 (Musella) describes warm compaction more in general. According to this patent, lubricants generally used for cold compaction, e.g. zinc stearate, can be used for warm compaction as well. In practice, however, it has proved impossible to use zinc stearate or ethylenebisstearamide (commercially available as ACRAWAX(copyright)), which at present are the lubricants most frequently used for cold compaction, for warm compaction. The problems which arise are due to difficulties in filling the die in a satisfactory manner.
U.S. Pat. No. 5,744,433 (Storstrom) discloses a method for making sintered products on the basis of a metal-powder composition containing a lubricant, which is an oligomer of amide type having a weight-average molecular weight Mw of 30,000 at the most and, preferably, at least 1,000 enabling the manufacture of compacted products having high green strength and high green density. The tested lubricants according to this patent may give problems such as cold welding and scratches may form on the ejected compacts. Also the risk of excessive tool wear cannot be eliminated.